1. Field of the Invention
The present invention relates to an ultrasonic motor and an electronic appliance equipped with an ultrasonic motor, and more particularly to an ultrasonic motor capable of controlling a drive of a moving body.
2. Description of the Related Art
An actuator employed as a drive source for various electronic appliances constitutes an essential element as drive means of a driven body. Generally, an electromagnetic type actuator including a magnet and a coil has been popularly employed. For example, a cross coil type actuator, a voice coil type actuator, or a stepping motor or the like has been used for an angular control of an indicator or a mirror to control the movement of a fixed angle and for driving of a driven body of a pick up (head) of a n information appliance.
Recently, an electomagnetic type actuator has attracted attention as an actuator based on a new principle and its application to various appliances including an indication instrument has been proposed as shown in Japanese Patent Publication Hei 4-64598, for example.
However, in case the electromagnetic type actuator is used, there has been a problem in that the actuator shows a poor response characteristic and a small holding torque and hence, a precision of a position control of the driven body in response to a dynamic input is deteriorated or the fluctuation of the position occurs due to an external vibration or the like. Furthermore, to hold the actuator at a specified position, it is necessary to keep an electric current to continue its flow. Accordingly, the current consumption is increased and heat is generated and hence, the use of the actuator in a vacuum becomes difficult. Furthermore, the actuator is liable to receive the influence of magnetism and generates magnetism by itself so that there arise problems such that the use of the actuator in a magnetic field is difficult and the actuator cannot be disposed in the vicinity of a magnetic medium or the like which dislikes magnetism. Furthermore, the actuator still has other problems in that the torque thereof is small and the miniaturization of the actuator is difficult in view of problems based on the operating principle and the construction, thus making an appliance equipped with the actuator to become large-sized.
Recently, in place of the above-mentioned electromagnetic type actuator, an ultrasonic motor has been used.
FIG. 4 shows the construction of such an ultrasonic motor.
As shown in FIG. 4, a disc-like vibrating body 6 is supported by a central shaft 5 which has its center fixedly secured to a support plate 4. The vibrating body 6 has a first surface thereof adhered to a piezoelectric element 7 and a second surface thereof provided with protrusions 6a which enlarge a vibration displacement of the vibrating body 6 and gives a rotating force to a moving body 8. A bearing 3 is mounted on the central portion of the moving body 8 and its center is guided by the central shaft 5. Furthermore, by pressurizing an inner race of the bearing 3 with a spring member 2, a contact pressure is generated between the protrusions 6a of the vibrating body 6 and a friction member 8a of the moving body 8. A vibrating wave excited in the vibrating body 6 due to the piezoelectric effect of the piezoelectric element 7 is converted to a rotational force of the moving body 8 by way of a friction force. A driven body 9 is fixedly secured onto the moving body 8 by means of an adhesion or a screwing and hence, the driven body 9 is driven corresponding to the rotation of the moving body 8.
This ultrasonic motor also has problems in that its lifetime is short because of a friction driving, the reliability of the actuator at the time of starting is low, and the response characteristic is sometimes excessively fast so that vibration is generated by noise or the like associated with a control signal. In particular, in a rotary-type ultrasonic motor with a moving body having a rotating angle of less than 360 degrees or in a linear-type ultrasonic motor, the contact range between the vibrating body and the moving body is restricted and hence, a biased wear is produced thus facilitating the deterioration of the motor characteristics. In this case, a start position, namely, an origin, an operation end point, and positions where a stop frequency is extremely high define wear boundaries so that the starting is deteriorated and the accuracy of position control is lowered. Furthermore, there is a problem in that a frequency tracking circuit becomes an inevitable requirement and it is necessary to make the phase difference of two signals vary at the time of changing the direction of the rotation to either a normal direction or a reverse direction and hence, a drive circuit becomes complicated and a circuit mounting portion becomes large-sized and expensive.
The present invention is provided for performing a drive control of a driven body using an ultrasonic motor as a drive source of an electronic appliance, wherein the ultrasonic motor is stopped at specified positions without consuming electric current and almost receiving no adverse influence of external vibration or the like by making use of a large holding torque which is a feature of the ultrasonic motor during a non-energized period. Furthermore, no heat is generated at this time, and the ultrasonic motor can be used in vacuum. Still furthermore, the ultrasonic motor does not receive the adverse influence of magnetism and does not generate magnetism by itself and hence, the ultrasonic motor is used in a magnetic field and enables the driving of a magnetic recording medium or its detection pick-up which dislikes magnetism without limiting a position or an arrangement of the ultrasonic motor.
In particular, an input signal transmitted to a control circuit which constitutes a drive command signal is made not to respond to noise components of the input signal in such a manner that frequency components which are equal to or exceed a specified frequency are eliminated by making the input signal pass through a filter, or the input signal is made not to respond to a minute fluctuation value, or the input signal takes on a mean value of the input signals. Accordingly, the vibration of a control system can be restricted in such a manner that a feature of the ultrasonic motor is that it exhibits high response characteristics and does not bring about any adverse effect to the contrary. Simultaneously, any unnecessary operation is eliminated so that wear of a contact portion between the vibrating body and the moving body is reduced thus prolonging the lifetime of the ultrasonic motor.
Furthermore, by intentionally making the ultrasonic motor be driven in a range larger than a drive range, the influence of the biased wear in the drive range is minimized, thus preventing deterioration of starting performance and lowering of an accuracy of position control.
Still furthermore, by varying the voltage or the frequency of the drive signal at the time of starting or stopping the ultrasonic motor, the amplitude of the vibrating body is gradually increased or decreased so that the wear of the contact portion between the moving body and the vibrating body by a sudden start or a sudden stop of the moving body is reduced thus prolonging the lifetime of the ultrasonic motor.